Sialic acids are found in a diverse group of glycoprotein oligosaccharides. At least six sialyltransferases are thought to be required to account for the variety of oligosaccharide structures observed. Three sialytransferases have been purified to homogeneity, and each has a strict acceptor specificity which limits its transfer of sialic acid from CMP-sialic acid to an oligosaccharide of defined structure. The unique specificities exhibited by these enzymes allows them to be used as specific reagents to modify the oligosaccharides of glycoproteins. A number of biological processes are mediated by the recognition of oligosaccharide structures which contain essential sialic acid residues. To date, purified sialyltransferases have been instrumental in understanding the molecular basis for the inactivation of a hepatic galactoside binding protein by neuraminidase, the adsorbtion of certain myxoviruses to erythrocyte receptors, and the role of sialic acid in the expression of the human M and N blood group antigens. In the proposed research, three additional sialytransferases which form the sequences SA alpha 2,3Gal Beta 1, 4GlcNZc, SA alpha 2, 6Gal Beta 1, 3GalNAc and SA alpha 2, 8SA found in glycoprotein oligosccharides will be identified and purified to homogeneity. Their enzymatic properties and substrate specificities will be established to deduce their role in oligosaccharide biosynthesis, and to determine their suitability to serve as specific reagents to examine the biological roles of sialic acids.